Since styrenic thermoplastic elastomers provide excellent mechanical strength, flexibility, weather resistance, ozone resistance, heat stability, and transparency when prepared as a composition, they have been utilized being molded into various kinds of cushioning materials, damping materials, sealing materials, grips, adhesives, and toys. Particularly, for the application uses of the thermoplastic elastomer compositions in direct or indirect contact with human bodies such as cushionings for beds and chairs and insoles, it is required that they have high flexibility and excellent compression set property (less compression set) and high moldability (fluidity) in combination in view of good cushioning properties upon actual use and productivity.
As compositions obtained by blending SEBS or SEPS as hydrogenated styrenic elastomers with a paraffin oil or the like capable of remarkably improving the flexibility, there have been proposed, for example, a gel composition including styrene-(ethylene/propylene)-styrene block copolymer and a nonaromatic extender liquid (Patent Document 1); a gel composition including a triblock type styrenic elastomer, a diblock type styrenic elastomer, and a softener (Patent Document 2); a gel composition including a high molecular weight triblock type styrenic elastomer, a low molecular weight triblock typo styrenic elastomer, and a softener (Patent Document 3) and a composition used for cable filling including a mineral oil or synthetic oil and, a radial hydrogenated SEBS synthetic rubber (Patent Document 4).
However, while the thermoplastic elastomer compositions disclosed in the patent documents described above are excellent in the flexibility and the compression set property, they are not always satisfactory in the fluidity and leave a room for improvement.
On the other hand, as a method of improving the fluidity of compositions, it may be considered to branch a polymer into a multi-arm type. As the compositions containing such multi-arm type polymer, there have been proposed, for example, an adhesive composition including a not uniformly branched block copolymer obtained by reacting a blend including two or more block copolymers selected from (A) B1-A1-B2—X, (B) A2-B3—X, and (C) B4—X (B1 to B4 each represent a conjugated diene block, A1 and A2 each represent a vinyl aromatic, hydrocarbon polymer block, and X represents an active terminal end of a living anion polymer) combined at a specific weight ratio, with a coupling agent that reacts by way of three or higher functions with the active terminal end of the living anionic polymerization or a mixture thereof, a tackifier resin, a process oil, and a stabilizer (Patent Document 5); and an adhesive composition including a multi-block copolymer of D-A-(B-A)n-Dx and/or (D-A-B)q-Y (where A represents a vinyl aromatic hydrocarbon polymer block, B and D each represent a hydrogenated conjugated diene polymer block, n is an integer of 1 to 5, x is 0 or 1, q is 2 to 30, Y represents a polyfunctional coupling agent, and the copolymer has from 9 to 35% by weight of a vinyl aromatic hydrocarbon content) and a tackifier resin (Patent Document 6). However, the block copolymers disclosed in Patent Document 5 or 6 do not satisfy the compression set property as the composition obtained by blending a paraffin oil and the like and leave a room for the improvement.